Deoxidation method of molten steel

ABSTRACT

METHOD OF DEOXIDIZING MOLTEN STEEL BY ADDING CALCIUM OR CALCIUM ALLOY TO THE LOWER PART OF THE MOLTEN STEEL IN A VESSEL AFTER OR DURING DEOXIDATION. BY INCREASING THE YIELD OF CALCIUM IN THE MOLTEN STEEL, FLOATATION OF SOME INCLUSIONS ARE RAPIDLY ACCELERATED AND THEN THOSE ARE EASILY SEPARATED, AND THE PRESENCE OF HARMFUL MACRO INCLUSIONS MAY BE DECREASED IN A GREAT QUANTITY.

April 20, 1971 YQSHIQ MlYAsHlTA ETAL 3 ,71,595

DEOXIDATION METHOD OF MOLTEN STEEL 4 Sheets-Sheet 1 Filed Oct. 10, 1968I 1/ 1 COO I r I April 20, 1971 Filed Oct. 10, 1968 FIG.2.

YOSHIO MlYASHlTA ETAL DEOXIDATION METHOD OF MOLTEN STEEL Oxygenconcentration in the molten steel de oxidation 4 Sheets-Sheet 2 0;Oxygen in the molte steel before I Behaviour of total oxygen deoxided ina normal method at t,

Calcium added at t Calcium added at t 2 Behaviour of free oxygendeoxided in a normal method at t 3 Behaviour. of total oxygen with 4Behaviour of free oxygen with (Difference between i and 2, and 3 and 4are the amount of oxygen in the molten steel) Total oxygen (7.) O

\ Calcium added Metallic Si l l l I l 4 6 Metallic Si 2 3 4 5 6 Timeafter deoxidation with Si (minutes) April 20, 11971 YOSHIO MIYASHITA Hmzoxxmvrxou mmnon 0F, MQLTEN STEEL Filed 001;. 10, 1968 4 Sheets-Sheet 3Metallic Si 0 Toml oxygen Im 2:8: n w w m m m mw mu O .0 0 0 h d M d O OO m o m m W F Time after deoxidation with Si (minutes) @L E 222 2 w 0Metallic Al April 1971 YOSHIO MIYASHITA A 3,575,695

DEOXIDA'I'ION METHOD OF MOLTEN STEEL Filed Oct. 10 1968 v 4 Sheets-Sheet4 F G'U g g Metallic Al 3 2 M-\; 0.10 2 3 0.04 E I Cl d s ac' a d d E a0.02 e

OCT-0N" oxygen o 1 l l l I O Time after deoxidcnion with Al (minutes)United States Patent O U.S. Cl. 75-51 6 Claims ABSTRACT OF THEDISCLOSURE Method of deoxidizing molten steel by adding calcium orcalcium alloy to the lower part of the molten steel in a vessel after orduring deoxidation. By increasing the yield of calcium in the moltensteel, floatation of some inclusions are rapidly accelerated and thenthose are easily separated, and the presence of harmful macro inclusionsmay be decreased in a great quantity.

to conduct experiments on the laboratory or industrial scale and topublish similar conclusions. The phenomenal effect of calcium on theform and quantitative variation of inclusions in steel is alsorecognized in a number of papers. However, Sims et al. do not admit theeffect of calcium by comparing a deoxidation with Ca-Si with one usingSi. A number of workers hold that calcium is not effective.

One of the reasons for such contrasting results is that calcium isconsidered unstable and its effective yield in the steel low because itsmelting point is 850 C. and its boiling point is 1440 C. Also it issimply vaporized when it is added to the surface of steel bath.Accordingly, various researches have been conducted to obtain improvedresults. For example, the use of Fe-Ca base alloy will decrease oxideinclusions, but its effectiveness is small considering the large amountof calcium addition (more than 0.2% as (a)), as is shown in Table 1 andthe final amount of inclusions after addition is large. Moreover,analyzing for inclusions as well as the reliability of the results arenot without problems.

TABLE 1.ANALYSIS OF OXIDE INCLUSIONS Fe-Ca Oxide inclusions,weight/percent Heat addition alloy FeO MnO SiOz A1203 2 oxide Number:

II None 0. 0045 0. 0013 0. 0120 0. 0074 0. 0252 III None 0. 0055 0. 00230. 0097 0. 0075 O, 0250 l. 5 0. 051 0. 0009 0. 0055 O. 0068 O. 0183 IVNone 0. 004 0. 003 0. 001 0. 017 0. 025

V None 0. 004 0. 0042 0. 0002 0, 018 0, 0262 VI None 0. 004 0. 001 0.0015 0, 014 0. 0205 VII None 0. 003 131' O. 0014 0. 016 0. 0204 l. 0 0.005 tr 0. 0001 0. 006 0. 0111 SUMMARY OF INVENTION This inventionrelates to a method of manufacturing steel having higher cleanliness byadding calcium or calcium allov to the molten steel and effectivelyimproving yield of calcium. This eliminates some non-metallic inclusionsfrom the molten steel and further effectively desulphurizes and degassesthe steel.

BACKGROUND OF INVENTION Previously Mn, Si, Al, Ti, and Cr have beenwidely used for the deoxidation of steel. Ordinarily, deoxidation isconducted in the ladle at the period of tapping. The amount of oxygen ina molten steel is generally higher than that being in equilibrium withmetallic Mn,

Si or A1 of said molten steel. That is, it has been difficult to obtainsteel with little inclusions and to keep the amount of inclusions tolower'value, no matter how much deoxidizer is added to keep the amountof oxygen low, because the deoxidation products formed in the moltensteel and the oxide inclusions formed by erosion of refractory materialsand slag dragged into said molten steel amountto a considerablepercentage. Moreover, deoxidized steel with Si, Al, Ti, and the liketends to contain harmful macro inclusions for the steel, so that thisoften results in defects even when the amount of inclusions in the steelare negligible.

Many researchers relating to deoxidation with calcium have beenconducted in order to solve such problems. It was known as early as 50years ago that calcium alloy is a powerful deoxidizing anddesulphurizing agent and that it can be used in refining steel.Goldschmidt published work which showed that the use of Ca-Si fordeoxidation eliminates oxygen and sulphur from molten steel and formmelting .slags which can easily be separated from the molten steel. Thisfundamental research led many others In work, on a large scale, toconfirm the effect of calcium alloy, the effect of calcium alloy isgenerally recognized with as much as 5.2 kg./ton addition. This is anundesirable amount both because of production cost and because of theaimed composition of the steel.

Finally, research on the deoxidation with calcium have been conducted byadding calcium or calcium alloy on the top of the molten steel. Becauseof the inferior yields of calcium resulting from its vaporisation, itbecomes effective only after a large amount has been added.

Vacuum melting methods, vacuum degassing methods and electro slagmethods are used in order to decrease inclusions in the steel. These allrequire complicated devices and are skilled work, which are thereforeunsuitable for mass-production.

The drawbacks to the use of calcium and some conclusions on its use maybe summarized as follows:

(1) The yield of calcium in the molten steel is quite low when calciumor calcium alloy is added for deoxidation purposes. This inferior yieldtends to deteriorate as the amount of the molten metal increases.

(2) Accordingly, small amounts of calcium addition are not effective.

(3) However, the effect after a great amount of calcium addition is notnoteworthy. (The decrease of oxide inclusion is small and its finalamount high.)

(4) Calcium is generally added to the steel in alloy form and itsquantity is naturally restricted in accordance with the aimed steelcompositions. Therefore, the addition of calcium in a large quantitywill not result in a significant effect.

(5) As the reaction of the calcium at the time of addition is not known,there is no certain method established for the preliminary deoxidationor the timing of the calcium addition.

(6) Because of the vaporation of the calcium on the bath surface and itsviolent reaction, with its low melting and boiling points, the amount ofvaporization or reaction cannot be controlled at the time of addition.

(7) Accordingly, there is no known deoxidizing method effective indecreasing inclusions or oxygen with a small amount of calcium.

(8) Improvement of the steel properties with calcium is not to beexpected because of the low yield of calcium in the molten steel.

Mn, Si, or Al may be used in deoxidation without calcium. When the valueof equilibrating oxygen with metallic Mn, Si or Al in the molten bath isquite low, the concentration of total oxygen in the steel (the sum offree oxygen and oxide inclusions, and the oxygen found by oxygenanalysis) is kept at a considerably high level. Because of inclusionsnot removable from the molten bath, it is difficult to keep the totaloxygen stable below 0.004% no matter how much deoxidizer is added. Owingto various conditions at the time of deoxidation, inclusions that remainin steel tend to be unstable and create macro inclusions which areharmful to properties of steel.

According to the present invention there is provided a method ofdeoxidising molten steel, wherein calcium or calcium-containing materialis added to the lower part of molten bath which has been subjected to apreliminary deoxidation step.

For a better understanding of the invention, reference will now be madeto the accompanying drawings, in which FIG. 1 illustrates an embodimentof the invention;

FIG. 2 shows graphs illustrating the decrease of oxygen in steel byusing the present invention;

FIGS. 3 and 4 are graphs illustrating the decrease in total oxygen withthe addition of calcium at the time of Si-deoxidation;

FIGS. 5 and 6 are graphs, similar to FIGS. 3 and 4, obtained fromanother series of tests (Al-deoxidation).

Referring now to FIG. 1, calcium alloy is added through hollow cylinderor tubing 3 to the lower part of the molten bath 5. The calcium alloy isstored in tank 1 and its flow is controlled by adjusting valve 2 as gasis blown into said cylinder. Refractory lining 4 surrounds cylinder ortubing 3, where it enters into the said bath 5. The calcium rises to thesurface rapidly and eliminates inclusions in said bath and prevents ordecreases the occurences of macro-inclusions. The requirement forpreliminary deoxidation is to have the value of dissolving oxygen belowthe predetermined value of oxygen in steel. This matter must beconducted prior to calcium addition or during addition of calcium.

The calcium, melting point 1440 C. and boiling point 1440 0., when addedto the lower part of the molten bath immediately liquifies or vaporisesitself and as it rises to the surface, it melts into the bath, .therebyreacting with free oxygen and oxide inclusions. Inclusions reacting withadded calcium rapidly rise to the surface as changes of its componentsare brought about and are eliminated so that the value of the totaloxygen decreases surely.

By adjusting the valve 2 at the bottom of the tank 1 as shown in FIG. 1,and regulating the amount of the calcium addition, a degree of reactioncan be regulated at the time of addition to the molten bath.

Several refractory hollow cylinders can be used simul taneously insteadof the single cylinder shown in FIG. 1.

Thus, the yield of calcium or calcium alloy added to said bath isexcellent. (According to our experiments it is more than 70% in somecases.)

An analysis of the remaining calcium in steel is shown in Table 2 whichis a comparison between a prior art addition of 0.1% calcium to thesurface of 50 kg. molten bath, and an addition of the same amount madeby the method of the present invention.

4 TABLE 2.CALCIUM ANALYSIS IS STEEL PERCENT Percent The prior art 0.0015The present invention 0.013

This shows that the amount of calcium to be added is small as comparedwith the prior art method. (In the prior art, more than 0.2% calcium wasused. According to this invention, 0.05% addition is quite sufficient.)The more the amount of the molten steel increases, the longer becomesthe contact period of calcium either in liquid or vapour form. In somecases the static pressure of the molten bath is higher than the vapourpressure of the calcium, thereby making the present method moreeffective than ever.

The calcium or calcium alloy required for the present invention may be apowder, granule or in small lump form. Besides the calcium or calciumalloy or calcium compound, magnesium, sodium or potassium or theiralloys and compounds may be added.

The calcium or calcium alloy is usually added to ladle containing themolten steel but it can also be added to open hearth, converted,electric furnace or mould.

Inert gases like argon or nitrogen gas may be used when the calcium orcalcium alloy is added. In the case of nitrogen addition, formation ofnitrides can be effectively achieved. Other additives aimed at theformation of nitrides can also be added along 'with the calcium orcalcium alloy. In this case, the partial pressure of oxygen in themolten bath contacting nitrogen gas is kept at a very low level bycalcium and nitrogen absorption in said bath is very effective.

Because of an exothermic reaction by addition of the calcium or calciumalloy, the deoxidizing agent can be added without lowering thetemperature.

The molten steel is stirred well as the calcium vaporizes and rises tothe surface of said bath, thereby making composition of said bathuniform.

As is indicated in Table 2, the contents of calcium remaining in steelis smaller, so that calcium has the possibility of improving propertiesof steel greatly.

Much of inclusions in the molten bath that have hitherto been unable torise to the surface of said bath are made to rise rapidly and in stablecondition by the addition of the calcium. The decrease in percentage ofinclusions is large, the final level of contents reached is small andstable.

By adding 0.05% calcium to 50 kg. molten steel which has beenpreliminarily deoxidized with Mn, Si or Al, it is possible to keep theconcentration of oxygen below 0.002%. By the prior art, it is diflicultto keep it below 0.005%. The composition of inclusions remaining insteel is made to change to one which prevents harmful macro inclusionsfrom being formed. Accordingly, the size of inclusions in a steel ingotis extremely small.

Any aimed composition of steel is possible to obtain easily as theamount of added calcium is small.

When this method is compared with the vacuum melting process, the vacuumdegassing process or the electro slag process, the apparatus isextremely simple requiring no skilled work and is therefore suited formass production.

The nature and effectiveness of the present invention will now be morefully discussed with reference to FIGS. 2 to 6.

FIG. 2 contains curves 1, 2, 3 and 4. O is the level of oxygen in themolten steel before deoxidation. Curve 1 shows the behaviour of totaloxygen, when deoxidation is effected in a normal manner at time T whilecurve 2 shows the behaviour of free oxygen under the same conditions. Incurve 3 calcium has been added at time T and the total oxygen is shownon the curve 3, while the behaviour of the free oxygen under the sameconditions is shown in curve 4. The difference between curves 1 and 2respectively and curves 3 and 4 respectively gives 5 contents of oxygenpresent as inclusions in the molten steel.

FIGS. 3 and 4 show the behaviour of total oxygen in electrolytic iron, 1kg., which has been melted in a high frequency furnace, with thetemperature kept at 1600 C. and to which is added 0.05 calcium after ithas been deoxidized with 0.3% silicon. In FIG. 3, the total oxygenreaches a constant value of 0.015% approximately in one minute aftercalcium has been added. Under the same conditions but without calciumaddition, it will also reach the set value of 0.015% in six to eightminutes after Si addition (see FIG. 4). In these experiments, themetallic Si present was 0.10% and the equilibrating oxygen was 0.015%.Free oxygen at the period of Si deoxidation is shown to decrease inapproximate equilibrium with metallic Si. That is to say, of the oxygenvalue of 0.065% in one minute after Si addition, most of it is SiO theprimary deoxidation product. This is found to rise rapidly to thesurface of molten bath and is eliminated by the calcium addition. Thesize of inclusions in steel after the calcium addition are extremelysmall, and its composition has been shown to be different from thecomposition prior to addition.

FIG. 4 shows an embodiment where calcium has been added to the bathafter Si constituting the primary deoxidation product had almost risento the surface of said bath and eliminated. In this example, it is foundthat a constant value of total oxygen, that is, about 0.015 is obtainedin about six minutes. Accordingly, in the process of Si-deoxidation, anaddition of calcium during Sideoxidation is desirable.

FIG. 5 shows that the total oxygen reaches 0.0016% and less if calciumaddition has been made. Under the same conditions, but without calcium,it reaches about 0.004% but not less in 6 to 8 minutes after thealuminum addition (see FIG. 6). In these experiments, the metallic Alwas 0.14% and the equilibrating oxygen with Al is about 0.0001%. In thecase of Al deoxidation, fine particles formed in the primary deoxidationcannot rise to the surface of said bath even when the free oxygendecreases. The value of total oxygen is thus kept at a considerablyhigher level than that of the free oxygen and does not decrease. Of theoxygen value of 0.046% in one minute after Al addition, most of it is A10 the primary deoxidation product. It rapidly rises to the surface ofsaid bath and is eliminated, when calcium is added. The value of thetotal oxygen tends to approach the value of free oxygen.

FIG. 6 shows an example where calcium addition is made to the moltensteel where considerable A1 0 remains as the primary product. Thisfigure indicates that oxygen rapidly decreases and the value of thetotal oxygen approaches the value of free oxygen.

In FIGS. 5 and 6, the size of inclusions in steel after calcium additionbecomes extremely small and its composition is difterent from thatbefore the addition made. A1 0 generally tends to cluster, but suchtendency is not to be found in inclusions to which calcium addition hasbeen made. This shows that the efiective addition of calcium preventsthe macro inclusions peculiar to A1 0 inclusions.

The use of calcium alloy, when calcium content of said alloy is equal,gave similar results.

The foregoing results have hitherto been unknown. These facts revealthat calcium addition to the molten bath after or during deoxidation soas to keep a level of free oxygen contents at a point below the finallydesired level of oxygen will decrease, rapidly and in considerablequantity, the primary deoxidation products.

FIGS. 3 to 6 show the results of small scale experiments with the moltensteel of 1 kg., but the same or a similar effect is found when theamount of molten metal is increased to that of the scale of industrialproduction. The method is eflective because the manner of addition keepsthe amount of calcium escaping into the atmosphere as small as possibleby keeping the contact period of calcium in liquid or vapor form longerafter it has been added to the lower part of the molten bath.Accordingly, the more the amount of the molten steel, the longer thecontact period. In some cases the static pressure of molten bath becomeshigher than the vapor pressure of the calcium, thereby making the methodmore elfective. This has been confirmed in large scale tests.

The amount of deoxidizing element needed to keep free oxygen below0.004% is, in the case of metallic Al about 0.0004%, and in the case ofmetallic Ti according to the work of Chipman et al. about 0.02%. That isto say, by adding calcium or calcium alloy to molten steel deoxidizedpreliminarily to make metallic Al 0.0004% and metallic Ti 0.02% thetotal oxygen can be decreased to below 0.004%. In this case, thecomposition of the inclusions change so that it prevents harmful macroin clusions in steel.

In a process of adding calcium or calcium-base alloy not only thedevices mentioned above are used, but also blowing and adding through apenetrating hole on the bottom of a vessel may be employed.

What we claim is:

1. In the steel deoxidation process wherein molten steel containing freeoxygen is deoxidized in a vessel by the addition of a deoxidizer to saidmolten steel to substantially eliminate said free oxygen and to formnon-metallic inclusions in said steel resultant from the deoxidation,the improvement comprising blowing into said molten steel after theaddition of said deoxidizer thereto about 0.05% by weight of said moltensteel of at least one calcium additive selected from the groupconsisting of calcium and calcium alloys, said calcium additive beingblown into the lower part of said vessel containing said molten steel bya blowing gas, whereby the amount and size of said nonmetallicinclusions in the steel are substantially reduced.

2. A method as defined in claim 1, wherein said molten steel isdeoxidized to substantially eliminate said free oxygen by addition of atleast one deoxidizer selected from the group consisting of manganese,silicon and aluminum.

3. A method as defined in any one of claims 1 and 2, wherein saidblowing gas is nitrogen or argon.

4. A method as defined in claim 3, wherein said calcium additive isblown through a refractory hollow cylinder.

5. A method as defined in any of claims 3, 4, 1 and 2, wherein saidvessel is a ladle into which the molten steel has been poured.

6. A method as defined in any one of claims 3, 4, 5, 1 and 2, whereinsaid calcium additive is calcium.

References Cited UNITED STATES PATENTS 2,049,004 7/1936 Flannery 7557X2,819,956 1/1958 Strauss 7557 3,157,492 11/ 1964 Matuschkovitz 75513,215,525 11/ 1965 Sprankle 7558X 3,269,828 8/1966 Hale 7558 3,467,1679/1969 Mahin 75-57X FOREIGN PATENTS 776,315 6/ 1957 Great Britain 755 1OTHER REFERENCES The Making, Shaping and Treating of Steel, US. Steel,7th edition, 1957, pages 396 and 397.

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant ExaminerUS. Cl. X.R.

* g;;g UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO,1 D t d 20,

Inventor) YOSHIO MIYASHITA et al It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 64 "researchers" should be deleted and replaced with"researches", as indicated in specification at page 2, line 20.

Column 3,- line 52 l440C. should be deleted and replaced with "850C. asindicated in specification at page 6, line 9.

Column 5, line 56 after "addition" insert "is" as indicated inspecification at page 10, line 12.

Signed and sealed this fi th day of September 1971 (SEAL) Attest:

ilgfifi lgmg sggmmm. Refit-Eu GOTTSCHALK g 1061' Acting Commissioner ofPatents

